This invention pertains to improved thermal stabilizers for non-clay wellbore fluids and more particularly to the use of viscosifiers, such as polysaccharides treated with calcined dolomite.
In the search for new sources of oil and gas, wells are drilled to greater depths than ever before thought possible. As greater depths are reached, higher formation temperatures are encountered. The low solids polymer drilling muds and completion and work-over fluids used in these applications cannot withstand these higher temperatures.
Currently several different polymers are used as viscosifiers (i.e., materials which increase the viscosity of aqueous solutions) in these applications, although all polymers are not used in each application, with the preferred polymers being polysaccharides of which the cellulosic type and more specifically hydroxyethyl cellulose (HEC) is particularly preferred. These polymers are excellent viscosifiers in reasonably low concentrations and cause little or no formation damage. Other polymers used in these applications include hydroxyalkylated guar gum derived from the guar plant (Cyamopsis tetragonolobus) and xanthan gum, a polysaccaride produced through fermentation by the microorganism Xanthomonas campestris. These polymers exhibit somewhat the same properties as HEC but are not acceptable in all phases of drilling, completions and workovers due to possible formation damage. Formation damage as described herein refers to the plugging of the pores of the producing formation in such a manner that oil or gas flow is impaired and recovery from the damage is often difficult or impossible.
Thermal stability is defined herein as the ability of an aqueous polymer solution, stabilized or unstabilized, to retain as much as possible of its original solution viscosity after exposure for 72 hours to a temperature of 200.degree. F. (93.degree. C.) If an unstabilized sample is treated at 200.degree. F. (93.degree. C.) in either fresh water or brine (3.5% NaCl) for 72 hours, its viscosity loss is generally in the 55-65 percent range.
Therefore an object of this invention is to improve the operating capability of polysaccharides, such as, hydroxyethyl cellulose so as to increase their thermal stability.
Other objects will become apparent to those skilled in the art upon a reading of the specification.
The current state of the art involves the use of magnesium oxide (magnesia) disclosed in U.S. Pat. No. 3,953,335. In this patent, it is specified that the amount of magnesia based on the polymer weight, which is required to stabilize the polymer, is about 15 percent.